Anodization is an electrolytic process in which the metal is made the anode in a suitable electrolyte. When electric current is passed, the surface of the metal is converted to a form of its oxide having decorative, protective or other properties. The cathode is either a metal or graphite, at which the only important reaction is hydrogen evolution. The metallic anode is consumed and converted to an oxide coating. This coating progresses from the solution side, outward from the metal, so the last-formed oxide is adjacent to the metal. The oxygen required originates from the electrolyte used.
Although anodizing can be used for other metals, aluminum is by far the most important. Magnesium can be anodized by processes similar to those used for aluminum. Zinc can be "anodized" but the process is not truly comparable, depending upon a high voltage discharge that produces a pitted semifused surface. Several other metals, including copper, silver, cadmium, titanium, and steel can be treated anodically for decorative effects.
Anodic oxide coatings on aluminum may be of two main types. One is the so-called barrier layer which forms when the anodizing electrolyte has little capacity for dissolving the oxide. These coatings are essentially nonporous; their thickness is limited to about 13 A/volt applied. Once this limiting thickness is reached, it is an effective barrier to further ionic or electron flow. The current drops to a low leakage value and oxide formation stops. Boric acid and tartaric acid are used as electrolytes for this process.
When the electrolyte has appreciable solvent action on the oxide, the barrier layer does not reach its limiting thickness: current continues to flow, resulting in a "porous" oxide structure. Porous coatings may be quite thick: up to several tens of micrometers, but a thin barrier oxide layer always remains at the metal-oxide interface.
Electron microscope studies show the presence of billions of close-packed cells of amorphous oxide through the oxide layer, generally perpendicular to the metal-oxide interface.
Sulfuric acid is the most widely used electrolyte, with phosphoric also popular. Anodic films of aluminum oxide are harder than air-oxidized surface layers.
Anodizing for decorative, protective and adhesive bonding properties has used strong electrolytes such as sulfuric acid and phosphoric acid. U.S. Pat. No. 2,703,781 employs a mixture of these two electrolytes.
U.S. Pat. No. 3,227,639 uses a mixture of sulfophthalic and sulfuric acids to produce protective and decorative anodic coatings on aluminum. Other aromatic sulfonic acids are used with sulfuric acid in U.S. Pat. No. 3,804,731.
As a post-treatment after anodization, the porous surface is sealed according to numerous processes to determine the final properties of the coating. Pure water at high temperature may be used. It is believed that some oxide is dissolved and reprecipitated as a voluminous hydroxide (or hydrated oxide) inside the pores. Other aqueous sealants contain metal salts whose oxides may be coprecipitated with the aluminum oxide.
U.S. Pat. No. 3,900,370 employs a sealant composition of calcium ions, a water-soluble phosphonic acid which complexes with a divalent metal to protect anodized aluminum or anodized aluminum alloys against corrosion. Polyacrylamide has been proposed as a sealant.
U.S. Pat. No. 3,915,811 adds an organic acid (acetic acid, hydroxy acetic acid, or amino acetic acid) to a mixture of sulfuric and phosphoric acids to form the electrolyte in preparation for electroplating the so-formed anodic aluminum coating.
U.S. Pat. No. 4,115,211 anodizes aluminum by A.C. or superimposed A.C. and D.C. wherein the electrolyte solution contains a water-soluble acid and a water-soluble salt of a heavy metal. The water-soluble acid may be oxalic, tartaric, citric, malonic, sulfuric, phosphoric, sulfamic or boric.
U.S. Pat. No. 3,988,217 employs an electrolyte containing quaternary ammonium salts, or aliphatic amines and a water-soluble thermosetting resin to anodize aluminum for protective, ornamental or corrosion resistant applications.
The advantages of anodized aluminum as a carrier for lithographic printing plates were early recognized. Processes employing as electrolytes sulfuric acid, phosphoric acid, mixtures of these, or either of these in succession have been proposed. Prior to anodizing the sheet may be roughened mechanically or chemically. The need for a subcoating prior to application as a photosensitive layer to impart adhesion to the coating and hydrophilicity to the non-image areas was recognized. U.S. Pat. No. 3,181,461 uses an aqueous alkaline silicate treatment following the anodization step.
U.S. Pat. No. 2,594,289 teaches (Col. 1, lines 42-54) that porous anodic films but not nonporous anodic films are suitable for lithographic purposes, "since the porous film confers a better water receptive surface to the non-image areas of the plate and allows image-forming material to anchor effectively to the surface by penetrating the pores."
U.S. Pat. No. 3,511,661, since disclaimed, describes aluminum sheet for a lithographic printing surface anodized in aqueous phosphoric acid having an anodic film with a cellular pattern of aluminum oxide having cells with porous openings of about 200 A to 700 A in average diameter and a surface with 10 to 200 mg per square meter of aluminum phosphate.
U.S. Pat. No. 3,658,662 describes the electrochemical silication of a cleaned, etched aluminum plate to achieve a measure of hydrophilization.
In U.S. Pat. No. 3,902,976 a conventionally anodized aluminum sheet is electrolytically post-treated in an aqueous solution of sodium silicate to form a hydrophilic abrasion-resistant and corrosion-resistant layer suitable as a support for a presensitized lithographic sheet.
U.S. Pat. No. 4,022,670 carries out anodization of aluminum sheets in an aqueous solution of a mixture of polybasic mineral acid such as sulfuric or H.sub.3 PO.sub.4 and a higher concentration of a polybasic aromatic sulfonic acid such as sulfophthalic acid to produce a porous anodic oxide surface to which a photosensitive layer may be directly applied.
There is described in U.S. Pat. No. 4,090,880, a two-step process whereby a cleaned aluminum sheet is first coated with an interlayer material such as alkali silicate, Group IV-B metal fluorides, polyacrylic acid, or alkali zirconium fluoride and then anodized conventionally in aqueous sulfuric acid. Enhanced shelf life when overcoated with diazo sensitizers is claimed.
U.S. Pat. No. 4,153,461 employs a post-treatment with aqueous polyvinyl phosphonic acid at temperatures from 40.degree. to 95.degree. C. after conventional anodizing to a thickness of at least 0.2.mu.. The treatment provides good adhesion of a subsequently applied light sensitive layer, good shelf life and good hydrophilization of non-image areas after exposure and development as well as long press runs.
Plates of the above construction, particularly when the light sensitive layer is a diazo compound have enjoyed considerable commercial success. Nevertheless, certain improvements would be desirable. These include freedom from occasional coating voids, occasional unpredictable premature image failure on the press, faster, more dependable roll-up on the press and freedom from other inconsistencies. Still greater press life is desirable as well as a process that would be more economical than conventional anodizing followed by a second operation of sealing or post-treating in preparation for coating with a light sensitive layer.
In the case of protective and decorative applications, improved corrosion resistance and production economy over known anodizing processes is desired.